PYROMETRY 471 with colored lights are sent from here to the furnace operators. The signals may be changed as frequently as necessary, and the measurements of temperature can be more reliable than when the indicators are placed in the furnace room. Automatic Signaling.—A recorder may be easily modified to accomplish automatic signaling. With the galvanometric recorder the chopper bar instead of depressing the galvanometer pointer onto the chart makes an electric contact either through the insulated extremity of the pointer or by pressing together a double spring contact. These contact points are carried upon an arm which may be swung across the scale and adjusted for any temperature range. The independent electrical circuits containing the contact points are thus opened and closed depending upon the deflection of the galvanometer pointer, and may be employed for any mode of electrical signaling. Galvanometric recorders have not been very successfully constructed to record and signal at the same time, but the potentiometric recorder is easily made to do this. The latter instrument may be fitted with two or more contact points on its slide wire disc which touch a stationary contact when the slide wire is turned. Figure 32 shows diagrammatic-ally the manner in which the movement of the slide wire serves to convey signals to the operator of a furnace. In addition to the colored signal lights the instrument may be equipped with a differential meter which indicates the departure of the furnace from the proper temperature. Such a mechanical arrangement for signaling does not interfere in any way with the mechanism for recording. Automatic Temperature Control.—Indicators or recorders fitted with means for closing electric contacts are made by the Bristol Co., Brown Instrument Co., and the Leeds & Northrup Co. Automatic control requires the expenditure of Slide Wires S and Si are Mounted on the same Shaft and are adjustable with respect to each other. Furnace Indicator and Signal Lamps Circuits for Operation of Furnace Indicator and. Signal Lumps from Curve Drawing Potentiometer Recorder. Thermocouple Connection sc c. ^ ^ \ 1 T.~C. Iff? K I , , 1 'SStd. Cell Votentiometcr Circuit FIG. 32.—Automatic, signal system for temperature control. more power than is required to operate signals, and relays are necessary between the controller and the furnace. Gas and oil valves are opened and closed by means of solenoid magnets, and electric switches are easily operated for electrically heated furnaces.